Generally, within the field of manufacture and assembly of circuit boards, electronic components are mounted at a specific position on a substrate, such as a printed circuit board (PCB), by a pick-and-place or component mounting machine. The electronic components are picked from component feeder magazines and positioned on the substrate, for mechanical and/or electrical connection to the substrate, by a work head, often referred to as a pick-up head or component mounting head.
Prior to placing components on a substrate, the substrate has typically been provided with a viscous medium in the form of solder paste or glue at accurately determined locations on the substrate at which the component is to be placed and attached to the substrate. The application of the viscous medium is typically provided through screen printing, dispensing or jetting. Such dispensing or jetting is typically provided through a work head, also referred to as a dispensing or jetting head.
Servo controlled linear motions of the work head may be provided in the x and y directions, or along the x- and y-axes, i.e. in the general, horizontal plane of the circuit board, and in the z direction, or along the z-axis, i.e. vertically and generally perpendicular to the plane of the circuit board. For a work head used for jetting, there may be no need for providing motion of the work head in the z direction, which however is vital for a dispensing and a component mounting head.
The continued miniaturization of electronic circuits continuously increases the demand for more accurate placement of components and viscous medium for attaching the components. In order to meet such demands and to also increase manufacturing speed, more powerful linear motors may be provided and preferably arranged closer to the work head to reduce or eliminate influence of motion transmission. However, a drawback of providing more powerful linear motors closer to the work head, is that excess heat is generated closer to the actual positioning location. This may result in thermal expansions of parts of the positioning system, which will have a detrimental impact on positioning placement accuracy.
In a conventional linear motor arrangement having a stationary motor member and a therewith co-operating movable motor member, the driving movable motor member is provided with a number of iron cores having thereon wounded copper cable, i.e. an electric conductor. The oppositely arranged stationary motor member is provided with a number of magnetic elements arranged in a row along an axis. Preferably, these magnetic elements are angularly adjusted or tilted, thereby providing a more precise and continuous linear motion. During operation, electric current flows back and forth through the electric circuit coils surrounding the cores, thereby generating heat which may result in thermal expansion of the moving motor member. Thus, a linear motor arrangement used in a positioning system that is intended for precise and fast linear motion generates a large amount of heat. The heat causes the moving motor member to expand, possibly bend, which may result in a strained connection with built-in stress to a thereto connected positioning unit, which in turn may cause deformation of said positioning unit. Deformations of the positioning unit may also arise from thermal deviations of the positioning unit per se, if the positioning unit is subjected to large temperature variations.
Such a deformation may result in changes in the relative positions and orientations of elements comprised in or carried by the positioning unit, such as the nominal centre of the positioning unit, a detector for detecting the position along the positioning axis, a camera for detecting reference markings on a substrate or the like, and one or more work heads, e.g. component mounting heads and/or dispensing heads. Such a change in relative position, i.e. offset, between different elements carried by the positioning unit is undesired, since it is likely to have a detrimental effect on the accuracy in the positioning of the work heads, and thereby result in a decrease in production accuracy or, as a result of correcting for inaccuracies, production speed.